What's the minimum number of different B&B's that I will need to stay at ? We usually like to stay at the same place all week, and use this as a central hub, but I'm guessing this will not be practical for completing the walk !

So over a 4-night holiday, will it be do-able with 2 nights in 2 different B&B's, or would you recommend 3 different places ?

Many people do the three peaks over several days. You can either do one per day, or 2 each day (repeating one of them).
If you can spare the time, doing one per day is good - especially if you add on a suplementary walk. For example walk over Whernside, and down into Kingsdale arriving in Dent. Then catch the Settle-Carlisle train back closer to where you are staying. Similarly walking over Pen-y-Ghent and Ingleborough can start or end at either Ribblehead or Horton-in-Ribblesdale stations. Ingleborough can also take in the show caves at White Scar, Ingleborough or the gaping gill winch (if you co-incide with the twice yearly opening).

So its very possible to say in the same place for the 2-4 nights you will need. If you wanted to stop in different places you will either need to carry all your stuff or pre-arrange with the B&B owners to collect it from the previous location. If your trip was mid-week this type of one night stay might be more possible. You will tend to find at weekends that most places only take 2night bookings.